Ferroelectric thin-film production method, voltage-application etching apparatus, ferroelectric crystal thin-film substrate, and ferroelectric crystal wafer

ABSTRACT

A ferroelectric thin-film production method produces a ferroelectric crystal thin film by using a ferroelectric crystal having first and second surfaces opposed to each other and having an etching rate of the first surface greater than that of the second surface and etching the first surface of the ferroelectric crystal. While etching, a predetermined voltage is applied to the ferroelectric crystal. When the etching progresses and the thickness of the ferroelectric crystal reaches a target value, the direction of polarization of the ferroelectric crystal are inverted and the progress of the etching automatically stops. Consequently, a ferroelectric crystal thin film extremely thin and uniform in thickness over a wide area can be produced.

TECHNICAL FIELD

The present invention relates to a ferroelectric thin-film productionmethod, a voltage-application etching apparatus used in theferroelectric thin-film production method, and a ferroelectric crystalthin-film substrate including a thing-film produced in the ferroelectricthin-film production method.

BACKGROUND ART

Conventionally, as a high-density, large-capacity and randomlyaccessible recording/reproducing apparatus, an optical disc apparatusand a HDD (Hard Disc Drive) apparatus are known.

In optical recording, data is recorded by using an optical pickup with alaser used as a light source to form a pit in a layer or layers of anorganic dye (or pigment) or a phase change material formed on a disc, orthe data is reproduced by using the fact that the reflectance of thelayer varies with or without the pit. Alternatively, the data may berecorded or reproduced by using a magneto optical effect. However, theoptical pickup is larger than the magnetic head of the HDD, which isinappropriate for high-speed reading. Moreover, the size of the pit isdefined by a light diffraction limit, so that it is considered that arecording density is limited to about 50 G bit/inch².

Moreover, in the longitudinal recording of magnetic recording, asrepresented by the HDD, a magnetic resistance (MR) head has beenpractically used, recently, by using giant magnetic resistance (GMR).Furthermore, its recording density is expected to be larger than that ofthe optical disc by using perpendicular magnetic recording. However,because of thermal fluctuation of magnetic record information and thepresence of a Bloch wall in a portion in which a code or sign isreversed or changed, and even if patterned media are used in view of theabove fact, the recording density is limited to 1 T bit/inch².

On the other hand, recently, the inventors of the present invention hasproposed a technology of a recording/reproducing apparatus using SNDM(Scanning Nonlinear Dielectric Microscopy) for nanoscale analysis of adielectric recording medium. In the SNDM, it is possible to increaseresolution related to measurement to sub-nanometer, by using anelectrically conductive cantilever (or probe) having a small probe onits tip, which is used for atomic force microscopy (AFM) or the like.Recently, a super high-density recording/reproducing apparatus has beendeveloped, wherein the apparatus records data into a recording mediumhaving a recording layer made of a ferroelectric material by using thetechnology of SNDM (Japanese Patent Application Laying Open NO.2003-085969).

DISCLOSURE OF INVENTION

By the way, in the super high-density recording/reproducing apparatus,polarization is locally reversed by the application of a voltage fromthe cantilever to the ferroelectric recording medium, and thispolarization reverse is used as record information. In the recordingoperation at this time, if a coercive electric field of theferroelectric material of the recording medium is Ec and a thickness ofthe ferroelectric material is d, a voltage V which satisfies a conditionof Ec≦V/d is applied to the ferroelectric material, which allows therecord information to be recorded by the polarization reverse.Therefore, as the thickness d of the ferroelectric material is smaller,an applied voltage for the recording operation can be lowered more.

Thus, in order to realize the lower voltage, it is desirable that thethickness d of the ferroelectric material has a thickness of severaltens nanometers, for example. Moreover, in order to produce theferroelectric recording medium, it is preferable to form a wafer inwhich an electrode and the ferroelectric material and the like arelaminated on the substrate, and to cut the wafer. In this case, in thewafer condition, it is necessary to uniform a thickness of theferroelectric material, with the several tens nanometers. This isbecause there is such a disadvantage that if the thickness of theferroelectric material of the ferroelectric recording medium which isproduced is not uniform, it is impossible to standardize the appliedvoltage which is applied to record the information, and it is difficultto produce the super high-density recording/reproducing apparatus.

However, in a sol-gel method, a spatter method, and the like which areconventionally used for the formation of a thin film, there is such atechnical problem that it is difficult or impossible to prepare auniform thin film related to the ferroelectric material. Morespecifically, even if flattening can be realized on the micrometerorder, not negligible unevenness or variation of the thickness may becaused on the nanometer order. In other words, even if it is possible toform the ferroelectric material suitable for the application of a Fe-RAM(Ferroelectric Random Access Memory) or the like, which is practicallyused on the micrometer order, there is such a technical problem that itis difficult or impossible to form the ferroelectric material suitablefor the application of the super high-density recording medium whichuses the ferroelectric material having a thickness on the nanometerorder.

As opposed to this, from the study performed by the inventors of thepresent invention, a thin-film formation method has been developed inwhich mechanical polishing, performed by a CMP (Chemical MechanicalPolishing) method, and dry etching are combined to make a thin film of aferroelectric single crystal. According to the thin-film formationmethod, it is found out that the ferroelectric substance can be thinnedto the order of 100 nm, and that uniform recording can be performed evenat 1.5 T bit/inch². However, in order to uniform the thicknessdistribution of the entire wafer having the radius of 3 inch on theorder of nanometer by the thin-film formation method, for example,strict process management is required, and there is such a technicalproblem that the cost greatly increases.

In order to solve the above-exemplified problems, it is therefore anobject of the present invention to provide a ferroelectric thin-filmproduction method which allows efficient production of a ferroelectricmaterial having a uniform thickness distribution, with a thickness ofthe order of sub-micrometers to several tens nanometers, for example, avoltage-application etching apparatus used in the ferroelectricthin-film production method, and a ferroelectric crystal thin-filmsubstrate including a thing-film produced in the ferroelectric thin-filmproduction method.

The present invention will be discussed hereinafter.

A ferroelectric thin-film production method of the present invention isprovided with: an etching process of dipping one surface in an etchingsolution to thereby etch the one surface, with respect to aferroelectric crystal which has the one surface and another surface thatface each other and in which an etching rate of the one surface isgreater than an etching rate of the another surface in such a conditionthat polarization directions are oriented in one direction; and avoltage applying process of applying a predetermined voltage between theone surface and the another surface.

According to the ferroelectric thin-film production method of thepresent invention, it is possible to produce the thin film of aferroelectric substance which has a uniform thickness and which hasflatness on the nanometer order. Incidentally, the “ferroelectricsubstance” in the present invention is a dielectric substance whichchanges the direction of spontaneous polarization by applying a voltageand which maintains the polarization direction even if the applicationof the voltage is subsequently stopped.

In particular, in the ferroelectric substance used in the presentinvention, the etching rate of the one surface is greater than theetching rate of another surface. In other words, the one surface hassuch a characteristic that the one surface is etched easier than anothersurface. This difference in the etching rate is determined, depending onthe direction of spontaneous polarization owned by the ferroelectriccrystal. The case where z-cut LiTaO₃ is used as the ferroelectriccrystal is discussed. In this case, if the ferroelectric crystal has thepolarization directions which are perpendicular to the one surface andanother surface and which are aligned throughout the entireferroelectric crystal, or if the ferroelectric crystal has a similarstructure to the above structure, a plus surface located on the plusside of the polarization direction is more difficult to be etched than aminus surface located on the minus side of the polarization direction.In this case, the minus surface is the one surface, and the plus surfaceis another surface. Incidentally, whether or not the ferroelectriccrystal has such a structure or characteristic varies depending on thetype of the ferroelectric crystal. In the present invention, it ispreferable to select and use the ferroelectric crystal having theabove-mentioned structure or characteristic.

Then, in the etching process, the one surface is etched by dipping theone surface of the ferroelectric crystal in the etching solution. Atthis time, it is preferable to take some measures so that a surface notto be etched (e.g. another portion other than the one surface) is notexposed to the etching solution, in the etching process.

Then, while the etching process is performed, the voltage applyingprocess is performed. In the voltage applying process, a predeterminedsize of voltage is applied between the one surface and another surfaceof the ferroelectric crystal. In this case, preferably, the one surfacewhich is exposed to the etching solution and another surface which isnot to be exposed to the etching solution are electrically insulated.

Here, the ferroelectric crystal has spontaneous polarization and hassuch a characteristic that the polarization direction of the crystal isreversed if an electric field having the opposite direction to thespontaneous polarization is applied from the exterior and the extent ofthe electric filed exceeds a constant value. The extent of a voltagewhich causes the reverse of the polarization direction is peculiar to amaterial, and an electric filed generated in the material by the voltageis referred to as a coercive electric field. The extent of the voltagewhich is applied to the ferroelectric crystal to cause the reverse ofthe polarization direction varies depending on the thickness of theferroelectric crystal. In the present invention, for convenience ofexplanation, it is assumed that the coercive electric field of theferroelectric crystal is “Ec”, and that the voltage applied when thepolarization direction of the ferroelectric crystal is actually reversedis a “polarization reverse voltage e”, while the explanation proceeds.For example, the coercive electric field Ec of LiTaO₃ (lithiumtantalate) described later is about 22 kV/mm. If the thickness of theferroelectric crystal is 100 nm, the polarization reverse voltage e isabout 2.2V.

In the voltage applying process, if a thickness to be desirably set onthe ferroelectric crystal is a desired or targeted thickness d, apredetermined voltage V applied to the ferroelectric crystal ispreferably set to be V=Ec×d. The electric field formed in theferroelectric crystal by the application of the voltage V issubstantially equal to the electric field formed in the ferroelectriccrystal by the application of the polarization reverse voltage e whenthe ferroelectric crystal has the desired thickness d. For example, ifthe desired thickness of the above-mentioned LiTaO₃ is desirably set to100 nm, the predetermined voltage V is set to 2.2V (i.e. 22 [kV/mm]×100[nm]).

If the etching progresses in the etching process, the ferroelectriccrystal melts from the one surface side, and the thickness is graduallyreduced. Along with this, the polarization reverse voltage e of theferroelectric crystal is gradually reduced. Then, when the thickness ofthe ferroelectric crystal reaches the desired thickness d, thepolarization reverse voltage e of the ferroelectric crystal issubstantially equal to the voltage V applied at that time. As a result,when the thickness of the ferroelectric crystal reaches the desiredthickness d, the polarization direction of the ferroelectric crystal isreversed. If the polarization direction of the ferroelectric crystal isreversed, the etching rate of the one surface is greatly reduced, sothat the progress of the etching is substantially stopped. Therefore, ifa uniform electric field is formed throughout the one surface of theferroelectric crystal by the predetermined voltage V which is applied tothe ferroelectric crystal, the thickness of the ferroelectric crystal isthe thickness d which is uniform throughout the entire surface.Incidentally, the application of an electric field to the etchingsurface (i.e. the one surface) of the substrate is performed, with theionized etching solution used as one electrode. In this case, theetching solution is spread throughout the one surface, so that it ispossible to uniformly perform the application of the electric field tothe entire one surface during the etching.

As described above, according to the ferroelectric thin-film productionmethod of the present invention, the thickness of the ferroelectriccrystal is set to the desired thickness by using such a characteristicthat the etching rate of the one surface is greatly reduced due to thereverse of the polarization direction. Thus, it is possible to form thethin film of the ferroelectric crystal which has a thickness on thesubmicrometer order or several tens nanometer order; for example, lessthan 1 μm. Moreover, even if the one surface of the ferroelectriccrystal has a large area size (e.g. about the same area size as that ofa wafer surface), it is possible to uniform the thickness throughout theentire surface.

In one aspect of the ferroelectric thin-film production method of thepresent invention, the ferroelectric crystal is a single crystal waferof a ferroelectric substance.

According to this aspect, it is possible to produce the ferroelectricsingle crystal wafer which has a thickness on the submicrometer order orseveral tens nanometer order and which has a uniform thicknessthroughout a large area, as described above. For example, the entiresurface of the wafer or a portion other than the periphery portionthereof can be made uniform in thickness. For example, if aferroelectric recording medium is produced, the thinned ferroelectriccrystal wafer is used. By this, it is possible to massively produce theferroelectric recording medium having the uniform thickness, andmass-produce the ferroelectric recording medium.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the ferroelectric crystal includes at least oneof LiNb_(x)Ta_(1−x)O₃ (0≦x≦1), M: LiNb_(x)Ta_(1−x)O₃ (0≦x≦1, M is adoping material), and K₃Li_(2−x)(Nb_(1−y)Ta_(y))_(5+x)O_(15+2x).

It is confirmed that there is a big difference in the etching rate dueto a difference in the polarization direction. By using theferroelectric crystal, it is possible to realize the above-mentionedferroelectric thin-film production method in which the etching rate ischanged by the reverse of the polarization direction, and it is possibleto derive an effect achieved by the production method. In other words,by using the ferroelectric crystal, it is possible to produce theferroelectric crystal thin film which has a thickness on thesubmicrometer order or several tens nanometer order and which has auniform thickness in a large area, inexpensively, certainly, and highlyaccurately.

Incidentally, as the material M doped to LiNb_(x)Ta_(1−x)O₃, forexample, MgO, ZnO, or the like can be used. Other materials can be dopedto LiNb_(x)Ta_(1−x)O₃, and can receive the above-mentioned variousbenefits.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the voltage which is applied to the ferroelectriccrystal in the voltage applying process is a direct current voltage.

According to this aspect, the etching is performed while a directcurrent (DC) voltage having a predetermined size of voltage is applied.By this, it is possible to produce the ferroelectric crystal thin filmwhich has a thickness on the submicrometer order or several tensnanometer order and which has a uniform thickness in a large area, asdescribed above.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the voltage which is applied to the ferroelectriccrystal in the voltage applying process is a pulse voltage.

For example, the continuous application of a voltage may cause anelectrochemical effect in the etching solution. This phenomenon likelyincreases the etching rate of another surface in which the etching rateis to be low originally (or the one surface after the polarization isreversed). As opposed to this, according to this aspect, it is possibleto etch the one surface (the one surface without the polarizationreversed), appropriately, without causing the electrochemical effectduring the etching.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the desired thickness d is less than 1 μm.

According to this aspect, it is possible to produce or mass-produce thethin film having a uniform thickness which is less than 1 μm. If theetching process and the voltage applying process are performed to theferroelectric crystal wafer, it is possible to produce the ferroelectriccrystal wafer having a uniform thickness which is less than 1 μm.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the etching solution includes hydrofluoric acid.

According to this aspect, it is possible to appropriately etch even amaterial which is relatively difficult to be etched like theferroelectric crystal.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the etching solution has conductivity.

According to this aspect, in applying a voltage between the one surfaceand another surface, the etching solution can be used as a medium toapply a voltage. For example, if a voltage is applied to the etchingsolution, it is possible to uniformly apply a voltage to the entire onesurface of the ferroelectric crystal which is exposed to the etchingsolution. Therefore, in the etching process and the voltage applyingprocess, it is possible to uniform the thickness of the ferroelectriccrystal and form a flat surface.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the etching process includes a heating process ofheating the etching solution.

According to this aspect, it is possible to promote the progress of theetching. Therefore, it is possible to produce the ferroelectric crystalthin film which has a thickness on the submicrometer order or severaltens nanometer order and which has a uniform thickness in a large area,in a shorter time.

In this case, the etching process (or the heating process) may include atemperature measuring process of measuring the temperature of theetching process in order to control the temperature of the etchingsolution.

Moreover, the etching process may include a refluxing process ofrefluxing the evaporated etching solution as the etching solution again,in order to prevent the reduction of the etching solution caused by theevaporation of the etching solution with heat.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the etching process includes a stirring processof stirring the etching solution.

According to this aspect, particularly during the progress of theetching, it is possible to keep the concentration of the etchingsolution uniform, and it is possible to preferably prevent thedispersion or variation of the progress rate of the etching due to adifference in the position on the one surface. Therefore, it is possibleto uniform the progress of the etching.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the etching process includes a filtrating processof filtrating the etching solution.

According to this aspect, it is possible to keep the high purity of theetching solution, by filtrating and removing the impurities in theetching solution. Therefore, it is possible to prevent the reduction ofthe progress rate of the etching and preferably prevent the dispersionor variation of the etching which is influenced by the impurities.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the ferroelectric thin-film production method isprovided with: an electrode forming process of forming an electrode in afilm shape on the another surface; a substrate connecting process ofconnecting the electrode and a substrate; and a polishing process ofpolishing the one surface of the ferroelectric crystal, and the etchingprocess and the voltage applying process are performed after theelectrode forming process, the substrate connecting process and thepolishing process.

According to this aspect, in the electrode forming process, an electrodeis formed on another surface of the ferroelectric crystal which is a rawmaterial. The electrode formed in this manner can be used as anelectrode in applying a voltage between the one surface and anothersurface in the above-mentioned etching process. Moreover, if theferroelectric crystal thin film produced in the production method of thepresent invention is used as a recording medium, the electrode formedhere can be also used as an electrode in applying an electric field, torecord the information into the recording medium or the like (which willbe discussed in detail later).

Then, in the substrate connecting process, the electrode and thesubstrate are connected. In this connection, as described later, anadhesive may be used to connect, or anodic bonding is used to connect,or another method may be used. In other words, a method is no object ifit can appropriately connect the electrode and the substrate. However,it is preferable to select a connecting method which providesexcellently close connection, which does not cause unnecessary stress bythe connection, which does not cause a deforming force which impairs theflatness of the surface of the ferroelectric crystal thin film, andwhich hardly causes deformation with time and deformation by temperaturechanges, in order to maintain the flatness of the surface of theferroelectric crystal thin film formed on the electrode.

For the substrate, a silicon substrate, a ferroelectric substance (e.g.a ferroelectric substance made of the same material as that of theferroelectric crystal which is used as the raw material in theferroelectric thin-film production method of the present invention), aglass substrate, or the like are used, for example. Alternatively, anymaterial other than these having strength as the support of theferroelectric crystal, as described later, can be used as the substrate.By this, it is possible to used the substrate as the support of theferroelectric crystal whose strength is reduced by thinning it.Therefore, the substrate preferably has a thickness to some degree, ascompared to the ferroelectric crystal. For example, if the thickness ofthe ferroelectric crystal is about 100 nm, the thickness of thesubstrate is preferably about 0.5 mm.

Then, in the polishing process, the one surface side of theferroelectric crystal on which the electrode and the substrate areformed is polished and thinned to an appropriate thickness (e.g. 10 μm).The polishing method may be a method of mechanically polishing by usinga surface grinding machine which uses a rotating whetstone, a rotatingdisc-shaped board, or the like, or may be so-called chemical polishing,such as simple etching. The ferroelectric crystal is roughly thinned inthe polishing process, and then thinned highly accurately in the etchingprocess and the voltage applying process, by which the entire thinningcan be performed efficiently, and the operation can be shortened.

In another aspect of the ferroelectric thin-film production methodprovided with the substrate connecting process, the substrate has adifference in a coefficient of thermal expansion of 50% or less, ascompared to the ferroelectric crystal.

In general, the volume (or area size) of various members, such as thesubstrate and the ferroelectric crystal, is expanded or contractedcaused by temperature changes. Therefore, even if the substrate and theferroelectric crystal are connected, the expansion or the like due tothe temperature changes possibly causes the exfoliation of theconnecting surface between the substrate and the ferroelectric crystal,or the damage of the substrate or the ferroelectric crystal, or thelike. However, according to this aspect, even if the substrate and theferroelectric crystal are both expanded (or contracted) due to thetemperature changes, that does not cause a big difference in the extentof their expansion or the like. Therefore, it is possible to prevent thedamage, or the exfoliation or the like of the connecting surface, asdescribed above, and it is possible to produce the ferroelectric thinfilm which is resistant to the temperature changes, relatively easily.This is a great advantage in the point that it is possible to realizethe ferroelectric recording medium which can perform stable recordingand reproduction operations, regardless of the temperature changescaused by the application of a voltage in the case where the recordingand the reproduction operations are performed by the application of thevoltage, as described later, for example.

More preferably, a coefficient of thermal expansion of the substrate anda coefficient of thermal expansion of the ferroelectric crystal have thesame or substantially the same value. By this, it is possible toeffectively prevent the damage, or the exfoliation of the connectingsurface, as described above, caused by the temperature changes upon theuse of the ferroelectric thin film.

In another aspect of the ferroelectric thin-film production methodprovided with the substrate connecting process, the substrate includes asame material as that of the ferroelectric crystal.

According to this aspect, regardless of the temperature changes, theextent of the expansion (or contraction) of the substrate and theferroelectric crystal is the same or substantially the same, so that itis possible to prevent the damage, or the exfoliation of the connectingsurface, more effectively.

As described above, in another aspect of the ferroelectric thin-filmproduction method provided with the substrate connecting process, it maybe constructed such that the substrate is a glass substrate, and thatthe electrode and the substrate are directly connected by anodicbonding, in the substrate connecting process.

By such construction, the anodic bonding is performed by applying avoltage of about several tens to several kV, between glass (i.e. thesubstrate herein) including alkali metal ions and metal or semiconductor(i.e. the electrode herein), at a temperature of about 200 to 400degrees Celsius in which the thermal diffusion of alkali metal ionsoccurs, with the metal side as an anode. Therefore, it is possible todirectly and firmly connect the electrode and the substrate, without anadhesive or the like, for example. Moreover, it is possible to realizeuniform adhesive strength on the adhesive surface and closely connect orbond the electrode and the substrate. At this time, the substrate is aglass substrate, and it is preferably a glass substrate includingcations, such as sodium ions and lithium ions (i.e. alkali metal ions),from the viewpoint that the anodic bonding is realized.

In another aspect of the ferroelectric thin-film production methodprovided with the substrate connecting process, it may be constructedsuch that a glass film is provided between the electrode and thesubstrate, and that the electrode and the substrate are connected byanodic bonding, in the substrate connecting process.

By such construction, even in the case of the substrate which does notinclude glass, it is possible to connect the substrate and theelectrode, appropriately and firmly, by the anodic bonding, by using theglass film. Incidentally, the glass film is preferably formed on theelectrode before the anodic bonding is performed. Then, by applying avoltage between the glass film and the substrate, the thermal diffusionof cations in the glass film occurs, to thereby connect the glass filmand the substrate. As a result, the glass film and the substrate arefirmly connected.

In another aspect of the ferroelectric thin-film production method ofthe present invention, the electrode and the substrate are connected byproviding an adhesive layer therebetween, in the substrate connectingprocess.

According to this aspect, it is possible to connect the electrode andthe substrate by using the adhesive layer, relatively easily and at lowcost. As the adhesive layer, an adhesive, such as epoxy resins, may besued, or a brazing material or the like may be used, as described later.

In an aspect of the ferroelectric thin-film production method in whichthe adhesive layer is provided in the substrate connecting process, asdescribed above, a conductive base layer which is highly adhesive isprovided at least one of between the adhesive layer and the electrode,and between the adhesive layer and the substrate, in the substrateconnecting process.

By such construction, even if the adhesive strength of the adhesivelayer is not strong or firm, it is possible to connect the substrate andthe electrode, firmly. In other words, even if the brazing material isused as the adhesive layer, for example, if there is provided theconductive base layer including titan and chromium which are highlyadhesive, it is possible to make a firm connection by the adhesion ofthe conductive base layer.

Incidentally, the electrode at this time is preferably the conductivebase layer which is highly adhesive.

In another aspect of the ferroelectric thin-film production method ofthe present invention, a smoothing process is performed to the onesurface after the one surface is polished.

According to this aspect, it is possible to smooth very small unevennesscause by the mechanical polishing in the polishing process, and improvean etching accuracy in the subsequent etching process. In the smoothingprocess herein, for example, CMP (Chemical Mechanical Polishing) or thelike may be used.

A voltage-application etching apparatus of the present invention is avoltage-application etching apparatus used in the etching process andthe voltage applying process in the above-mentioned ferroelectricthin-film production method of the present invention (including itsvarious aspects), the voltage-application etching apparatus providedwith: a container to dip the one surface in the etching solution; asealing device for sealing a portion other than the one surface from theetching solution; a power supply for applying the predetermined voltagebetween the one surface and the another surface; a first connectingdevice for electrically connecting one output terminal of the powersupply with the one surface; and a second connecting device forelectrically connecting another output terminal of the power supply withthe another surface.

According to the voltage-application etching apparatus of the presentinvention, it is possible to appropriately perform the etching processand the voltage applying process in the above-mentioned ferroelectricthin-film production method of the present invention.

Specifically, the ferroelectric crystal (the one surface thereof) isdipped in the etching solution. The etching solution is preferablystored in (or fills) the container. Then, the predetermined voltage isapplied between the one surface and another surface of the ferroelectriccrystal from the power supply. At this time, a voltage is supplied tothe one surface by the first connecting device. The first connectingdevice may be constructed to dip the electrode in the etching solutionhaving conductivity, as described later, for example. According to thisstructure, a voltage is applied to the one surface of the ferroelectriccrystal through the etching solution. If the one surface of theferroelectric crystal is exposed to the etching solution throughout theentire surface, a voltage is uniformly applied to the entire one surfaceof the ferroelectric crystal. As a result, it is possible to form auniform electric field in the ferroelectric crystal. By this, it ispossible to produce the ferroelectric crystal thin film having a uniformthickness.

On the other hand, a voltage is supplied to another surface by thesecond connecting device. As the second connecting device, for example,the electrode formed on the above-mentioned ferroelectric crystal (onanother surface side) may be used. Specifically, the electrode and thepower supply may be connected with a lead wire or the like.

After that, as described above, the etching progresses until theferroelectric crystal has the desired thickness. The polarizationdirection is reversed when the strength of the electric field caused bythe applied voltage exceeds the coercive electric field of theferroelectric crystal, and the progress of the etching is practicallystopped.

In particular, the etching solution is not exposed to a portion otherthan the one surface of the ferroelectric crystal, by virtue of thesealing device. Therefore, it is possible to appropriately etch the onesurface to be originally etched. The sealing device may be a packingincluding an O ring or the like, as described later, or may be a sealingcan, a sealing film or the like which covers the portion other than theone surface, for example.

As described above, according to the voltage-application etchingapparatus of the present invention, it is possible to appropriately etchanother surface of the ferroelectric crystal while a voltage is applied.Therefore, if the predetermined voltage is applied on the basis of thedesired thickness of the ferroelectric crystal and the coercive electricfield of the ferroelectric crystal, as described above, it is possibleto produce the ferroelectric crystal having the desired thickness. Inparticular, it is possible to realize the thickness on the submicrometerorder or several tens nanometer order, and it is possible to produce theferroelectric thin film having a uniform thickness in a large area,relatively easily.

Incidentally, in response to the various aspects of the etching processand the voltage applying process of the above-mentioned ferroelectricthin-film production method of the present invention, thevoltage-application etching apparatus of the present invention can adoptvarious aspects.

In one aspect of the voltage-application etching apparatus of thepresent invention, the sealing device is an acid-resistant O ring whichis pressed onto a periphery portion on the one surface or an outer edgeof the one surface.

According to this aspect, it is possible to relatively easily fixanother surface not to be exposed to the etching solution, selectively.

In another aspect of the voltage-application etching apparatus of thepresent invention, it is further provided with a heating device forheating the etching solution.

According to this aspect, it is possible to promote the progress of theetching. Therefore, it is possible to produce the thin film having theferroelectric crystal with the desired thickness in a shorter time.

In this case, the etching device (or the heating device) may include atemperature measuring device for measuring the temperature of theetching process in order to control the temperature of the etchingsolution.

Moreover, the etching device may include a refluxing device forrefluxing the evaporated etching solution as the etching solution again,in order to prevent the reduction of the etching solution caused by theevaporation of the etching solution with heat.

In another aspect of the voltage-application etching apparatus of thepresent invention, it is further provided with a stirring device forstirring the etching solution.

According to this aspect, particularly during the progress of theetching, it is possible to keep the concentration of the etchingsolution uniform, and it is possible to preferably prevent thedispersion or variation of the progress rate of the etching due to adifference in the position on the one surface. Therefore, it is possibleto uniform the progress of the etching.

In another aspect of the voltage-application etching apparatus of thepresent invention, it is further provided with a filtrating device forfiltrating the etching solution.

According to this aspect, it is possible to keep the high purity of theetching solution, by filtrating and removing the impurities in theetching solution. Therefore, it is possible to prevent the reduction ofthe progress rate of the etching and preferably prevent the dispersionor variation of the etching which is influenced by the impurities.

A ferroelectric crystal thin-film substrate of the present invention isprovided with: a substrate; an electrode formed on the substrate; and aferroelectric crystal which is formed on the electrode and which is lessthan 1 μm in thickness, wherein an area size of an entire surface of thesubstrate is equal to or greater than 10 mm².

According to the ferroelectric crystal thin-film substrate of thepresent invention, the ferroelectric crystal thin film is 1 μm inthickness and has a uniform thickness in a large area of 10 mm² or more.Thus, it is possible to use the ferroelectric crystal thin film as thematerial of a ferroelectric recording medium, and by this, it ispossible to mass-produce the ferroelectric recording medium.

Here, the ferroelectric recording medium has a recording layer made ofthe ferroelectric crystal thin film. Then, information is recorded byapplying a voltage large enough to form an electric field which exceedsthe coercive electric field to the ferroelectric crystal thin film tothereby reverse the polarization direction of the ferroelectric crystalthin film. Therefore, if the ferroelectric crystal thin film is thin, itis possible to reduce a voltage required for the reverse of thepolarization (i.e. required for the information recording). In addition,if the thickness of the ferroelectric crystal thin film is uniform, itis possible to make the voltage required for the information recordingconstant. If the voltage required for the information recording can bereduced, it is possible to reduce the power consumption of a recordingapparatus for recording the information into the ferroelectric recordingmedium. Moreover, if the voltage required for the information recordingcan be made constant, it is possible to unify an applied voltage for theinformation recording, on the recording apparatus. If the appliedvoltage can be unified, it is possible to standardize the appliedvoltage, for example, and it is possible to realize thecommercialization of the recording apparatus. Moreover, by using theabove-mentioned ferroelectric recording medium, the polarization can bereversed in a very small size on the nanometer order. Thus, it ispossible to realize a super high density recording/reproducing apparatuswith SNDM used as a reproduction method.

Incidentally, the ferroelectric crystal thin-film substrate of thepresent invention is preferably produced in the above-mentionedferroelectric thin-film production method of the present invention(including its various aspects).

In one aspect of the ferroelectric crystal thin-film substrate of thepresent invention, the substrate is a glass substrate, and the substrateand the electrode are directly connected by anodic bonding.

According to this aspect, it is possible to realize the ferroelectriccrystal thin-film substrate in which the substrate and the electrode areconnected, uniformly, closely, and firmly. Moreover, as explained in theabove-mentioned ferroelectric thin-film production method of the presentinvention, a difference in the coefficient of thermal expansion betweenthe substrate and the ferroelectric crystal thin film is preferably 50%or less. By this, it is possible to prevent the damage, the exfoliationof the connecting surface, or the like. Moreover, the substrate and theferroelectric crystal thin film may include the same material.

In another aspect of the ferroelectric crystal thin-film substrate ofthe present invention, in the ferroelectric crystal, polarizationdirections are perpendicular to the surface and oriented in a samedirection.

According to this aspect, in the case where the ferroelectric crystalthin-film substrate of the present invention is used for theferroelectric recording medium, it is unnecessary to separately performa format process to orient the polarization directions in the samedirection, or the like.

Specifically discussing, in the case where predetermined information isrecorded into the ferroelectric recording medium, the following processis performed; namely, the polarization direction is reversed when a databit corresponding to the information is “1”, and the currentpolarization direction is maintained when the data bit is “0”. In orderto realize this process, the polarization directions of theferroelectric crystal thin film of the recording medium are preferablyoriented in the same direction at an initial stage before theinformation is recorded. According to the present invention, thepolarization directions are oriented in the same direction in advance,so that it is possible to satisfy the requirement.

Moreover, the information recording into the ferroelectric recordingmedium is performed by a change in the polarization direction. Forexample, in order to record binary information by the change in thepolarization direction, the domain structure of a recording surface ispreferably a 180 degree domain structure. According to the presentinvention, the polarization axis of the ferroelectric crystal thin filmis perpendicular to the one surface of the ferroelectric crystal thinfilm. Thus, if the one surface is the recording surface, the domainstructure of the recording surface is the 180 degree domain structure,so that it is possible to satisfy the requirement.

As explained above, according to the ferroelectric thin-film productionmethod of the present invention, it is provided with: the etchingprocess; and the voltage applying process. Therefore, it is possible toproduce the thin film of the ferroelectric crystal having a uniformthickness on the submicrometer order or several tens nanometer order,relatively easily and efficiently, by appropriately adjusting orcontrolling a voltage which is applied upon the etching. Moreover, evenin the case of the ferroelectric crystal having a large area size, suchas a wafer, it is possible to produce the thin film of the ferroelectriccrystal having a uniform thickness throughout the entire large area, byperforming the etching while applying a predetermined voltage. Then, bythe production method of the present invention, it is possible tomass-produce the ferroelectric recording medium by using a ferroelectriccrystal wafer which is extremely thin and which has a flat surface.

Moreover, according to the voltage-application etching apparatus of thepresent invention, it is provided with: the container; the sealingdevice; the power supply; the first connecting device; and the secondconnecting device. Therefore, it is possible to appropriately performthe etching process and the voltage applying process in theabove-mentioned ferroelectric thin-film production method of the presentinvention.

Furthermore, according to the ferroelectric crystal thin-film substrateof the present invention, it is provided with: the substrate; the powersupply; and the ferroelectric crystal. Therefore, it is possible torecord the information at a relatively low voltage. Moreover, since thedielectric substance as being the recording layer has a uniformthickness, it is possible to prevent the variation of the appliedvoltage necessary upon the recording, and it is possible to standardizethe applied voltage. By this, it is possible to use the ferroelectriccrystal thin-film substrate as a recording medium to realize super highdensity ferroelectric recording.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing an entire producing process of anembodiment according to the ferroelectric thin-film production method ofthe present invention;

FIG. 2 is a cross-sectional view conceptually showing a ferroelectriccrystal used in the embodiment according to the ferroelectric thin-filmproduction method of the present invention;

FIG. 3 is a cross-sectional view conceptually showing the ferroelectriccrystal after an electrode forming process, in the embodiment accordingto the ferroelectric thin-film production method of the presentinvention;

FIG. 4 is a cross-sectional view conceptually showing one aspect ofsubstrate connection by a substrate connecting process, in theembodiment according to the ferroelectric thin-film production method ofthe present invention;

FIG. 5 is a cross-sectional view conceptually showing another aspect ofsubstrate connection by the substrate connecting process, in theembodiment according to the ferroelectric thin-film production method ofthe present invention;

FIG. 6 are cross-sectional views conceptually showing another aspect ofsubstrate connection by the substrate connecting process, in theembodiment according to the ferroelectric thin-film production method ofthe present invention;

FIG. 7 is a cross-sectional view conceptually showing another aspect ofsubstrate connection by the substrate connecting process, in theembodiment according to the ferroelectric thin-film production method ofthe present invention;

FIG. 8 is a cross-sectional view conceptually showing an aspect of apolishing process, in the embodiment according to the ferroelectricthin-film production method of the present invention;

FIG. 9 are cross-sectional views conceptually showing the basicstructure of a voltage-application etching apparatus used in an etchingprocess, in the embodiment according to the ferroelectric thin-filmproduction method of the present invention;

FIG. 10 are schematic diagrams conceptually showing the progress of theetching process, in the embodiment according to the ferroelectricthin-film production method of the present invention;

FIG. 11 is a graph showing a pulse voltage applied in the etchingprocess, in the embodiment according to the ferroelectric thin-filmproduction method of the present invention;

FIG. 12 is a perspective view conceptually showing a dielectricrecording medium prepared by using the thin film produced by theembodiment according to the ferroelectric thin-film production method ofthe present invention; and

FIG. 13 is an explanatory diagram conceptually showing a probe portionof a dielectric recording apparatus for recording information into thedielectric recording medium or for reproducing the information.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiments of the present invention will be explainedwith reference to the drawings. Incidentally, the ferroelectricthin-film production method explained below is intended to produce aferroelectric thin film (hereinafter referred to as a “thin film”, asoccasion demands) having a thickness thin enough to use a dielectricrecording medium, for example, mainly from a ferroelectric crystal.

At first, with reference to FIG. 1, an entire producing process relatedto the ferroelectric thin-film production method in this embodiment willbe discussed. FIG. 1 is a flowchart showing the entire producing processof the ferroelectric thin-film production method in the embodiment.

As shown in FIG. 1, the ferroelectric thin-film production method inthis embodiment is provided with: an electrode forming process (stepS11); a substrate connecting or bonding process (step S12); a polishingprocess (step S13); and an etching process (step S14).

In the electrode forming process, an electrode is formed on one surface(another surface) of a ferroelectric crystal wafer with a thickness of400 μm, for example.

In the substrate connecting process, the electrode formed in the stepS11 is connected or bonded to a substrate.

In the polishing process, the ferroelectric crystal is mechanicallypolished to be on the order of 10 μm in thickness, for example.

In the etching process, the ferroelectric crystal is dipped in anetching solution to perform etching. Then, during the etching, apredetermined size of voltage is applied to the ferroelectric crystal.By this, the ferroelectric crystal is thinned to be less than 500 nm inthickness, for example.

Particularly in the embodiment, in the etching process in the step S14,by using the change of an etching rate caused by the polarizationreverse of the ferroelectric crystal, it is possible to make theferroelectric crystal be less than 500 nm in thickness while keeping itin the wafer condition. Moreover, it is also possible to flatten theetching surface over a wide range. The detail of the etching processwill be discussed later.

Hereinafter, each process shown in FIG. 1 will be discussed in moredetail.

FIG. 2 shows a wafer of a ferroelectric single crystal, which is the rawmaterial of the ferroelectric crystal used in the ferroelectricthin-film production method in the embodiment. As shown in FIG. 2, thewafer of the ferroelectric single crystal (e.g. 100 mm in diameter) isused as the raw material of a ferroelectric crystal 100. Theferroelectric crystal 100 has spontaneous polarization Ps, and thedirection of the polarization (i.e. orientation) is directed from asurface 101 (one surface) to a surface 102 (another surface). In otherwords, the same state is realized as such a state that negative chargesare generated on the surface 101 side and positive charges are generatedon the surface 102 side. Then, in the ferroelectric crystal 100, thereis a big difference in the etching rate of the surface 101 and thesurface 102, due to the difference of plus (positive) and minus(negative) in the polarization axis direction.

As the ferroelectric crystal 100, there are LiNb_(x)Ta_(1−x)O₃ (0≦x≦1),M: LiNb_(x)Ta_(1−x)O₃ (0≦x≦1, M is a doping material, such as MgO andZnO), K₃Li_(2−x)(Nb_(1−y)Ta_(y))_(5+x)O_(15+2x), and the like. Forexample, in the above-mentioned LiTaO₃ and LiNbO₃ or the like, theetching rate of the surface 102 (i.e. the surface on the side thatpositive charges are generated) is smaller than that of the surface 101(i.e. the surface on the side that negative charges are generated), sothat the surface 102 is more difficult to etch than the surface 101.

However, materials used as the ferroelectric crystal 100 are not limitedto the above materials, and any materials may be used if having adifference in the etching rate due to a difference in the polarizationdirection.

Moreover, the ferroelectric crystal 100 preferably maintains uniformcrystallinity from the inside of the crystal to the surface (i.e. thesurfaces 101 and 102). Then, as shown in FIG. 2, all the polarizationaxes are preferably perpendicular to the surfaces and face in the samedirection. By this, the thin film, produced by the ferroelectricthin-film production method in this embodiment, can be used for aferroelectric recording medium, relatively easily, as described later.

FIG. 3 shows the “electrode forming process” in FIG. 1. As shown in FIG.3, in the electrode forming process, a film-shaped electrode 110 isformed on the surface 102 of the ferroelectric crystal 100.Incidentally, the surface 102 on which the electrode 110 is formed is asurface with a low etching rate, due to the polarization directioncurrently set on the ferroelectric crystal 100.

As the electrode 110, noble metal, such as Au and Pt, a metal material,such as Al, Cr, and Ti, or a conductive oxide, such as LASCO(La_(0.5)Sr_(0.5)CoO₃) and SrRuO₃, is used. The electrode 110 is formedon the surface 102 of the ferroelectric crystal 100 in a sputteringmethod or a deposition method, or the like. The sputtering method ordeposition method can use the respective conventional technologies.

At this time, a surface on which the electrode 110 is formed (i.e. aconnecting surface of the electrode 110 and the surface 102) isperpendicular to the polarization axis of the ferroelectric crystal.

Incidentally, the electrode 110 functions as an etching electrode forapplying a voltage to the ferroelectric crystal, in the subsequentetching process. Moreover, if the ferroelectric thin-film produced inthe production method in this embodiment is used as a ferroelectricrecording medium, the electrode 110 is used as an electrode in applyinga voltage to the recording medium.

Next, FIG. 4 to FIG. 7 show the “substrate connecting process” inFIG. 1. For example, there are four methods in the method of substrateconnection in the substrate connecting process. FIG. 4 to FIG. 7 showfour specific examples of the substrate connecting process using therespective four methods.

As shown in FIG. 4, the first specific example of the substrateconnecting process is to use an adhesive 111 to connect or bond theelectrode 111 and a substrate 120. As the adhesive 111, for example,epoxy resin and acrylate resin or the like may be used. By this, it ispossible to bond the electrode 110 and the substrate 120, relativelyeasily and at low cost.

In this case, as the substrate 120, a silicon substrate, a glasssubstrate, or a ferroelectric substance made of the same material as ordifferent material from the ferroelectric crystal 100, or the like canbe used. The substrate 120 will make the support of the ferroelectriccrystal 100, so that the thickness of the substrate 120 is preferablythicker than that of the ferroelectric crystal 100 (the thinnedferroelectric crystal 100 after the etching process). For example, thefinal thickness (desired thickness) after the etching process of theelectrode 110 is 100 nm, the thickness of the adhesive 111 is 0.5 μm,and the thickness of the substrate 112 is about 0.5 mm.

As shown in FIG. 5, the second specific example of the substrateconnecting process is to use a brazing material 113 to bond theelectrode 111 and the substrate 120. At this time, as the electrode 110,a conductive base layer 112 is used which is highly adhesive. Inaddition, the conductive base layer 112 is also provided between thebrazing material 113 and the substrate 120.

As the conductive base layer 112, it is preferable to use Al, Ti, andCr, which are highly adhesive. By this, it is possible to bond thebrazing material 113 and the conductive base layer 112, or the substrate120 or the like, more certainly. Preferably, the conductive base layer112 may be substantially 30 nm thick; however, the present invention isnot limited to this. It is enough if it is possible to bond the brazingmaterial 113 and the conductive base layer 112, or the substrate 120 orthe like. Moreover, the conductive base layer 112 can be also formed onthe ferroelectric crystal 100 or the substrate 120, in the sputteringmethod or deposition method, as in the above-mentioned electrode 110.

Then, the conductive base layer 112 which is provided between theferroelectric crystal 100 and the brazing material 113 becomes theelectrode 110, and is used as the electrode in the etching process andthe electrode of the dielectric recording medium.

Moreover, the brazing material 113 preferably includes Au and Sn, forexample. These Au and Sn are accumulated, separately or in an alloycondition, on the connecting surface in a thin-film shape, by using thesputtering method or the like. Then, in order to bond the connectingsurface, at least the connecting surface is heated to about 300 degreesCelsius, to thereby bond the conductive base layer 112 and the brazingmaterial 113, and the brazing material 113 and the substrate 120 (i.e.the surface on which the conductive base layer 112 is formed).

As shown in FIG. 6, the third specific example is to bond the electrode110 and the substrate 120 by anodic bonding. In this case, a glass film114 is provided between the electrode 110 and the substrate 120.

Namely, as shown in FIG. 6(a), the glass film 114 is formed on a surfaceopposite to the connecting surface with the ferroelectric crystal 100,out of the electrode 110. As the glass film 114, it is preferable to useglass including alkali metal of Na, Li, or the like, such as pyrex(registered trademark) glass, for example.

Then, as shown in FIG. 6(b), after the substrate 120 is further closelyattached to the glass film 114, it is heated to about 400 degreesCelsius, and a voltage of about 600V is applied between the glass film114 and the substrate 120. At this time, the glass film 114 side ispreferably set to a cathode. By this, alkali metal ion, such as Na ionand Li ion, is thermally diffused and displaced to the vicinity of aboundary surface of the substrate 120, to thereby make a connectionwhich has a firm, uniform adhesive strength and which is highlyreliable. Moreover, it is possible to firmly connect or bond thesubstrate 120 and the electrode 110, without an adhesive layer (i.e. theadhesive 111, the conductive base layer 112, and the brazing material113) as shown in FIG. 4 and FIG. 5.

On the other hand, as shown in FIG. 7, the fourth specific example ofthe substrate connecting process is to use a glass substrate 121including alkali metal, such as Na, to thereby directly perform theanodic bonding between the glass substrate 121 and the electrode 110. Bythis, without using the glass film 114, as in FIG. 6, it is possible toconnect or bond the electrode 110 and the glass substrate 121, by theanodic bonding, more firmly, more uniformly, and more closely.

Incidentally, in the fourth specific example in which the glasssubstrate 121 is used for the anodic bonding, the electrode 110preferably includes a metal material, such as Al, Cr, and Ti, forexample.

Incidentally, in the anodic bonding as shown in FIG. 6 and FIG. 7, it ispreferable to consider thermal expansion or the like of theferroelectric crystal 100, the electrode 110, the glass film 114, thesubstrate 120, and the glass substrate 121 or the like, caused by theheating at about 400 degrees Celsius and the application of a voltage ofabout 600V. In other words, it is preferable to consider the positioningof the substrate 120, or the like, for example, so as to appropriatelyconnect or bond the substrate 120 and the glass film 114, or the glasssubstrate 121 and the electrode 110, even by the thermal expansion.

In addition, it is preferable to use the substrate 120 made of amaterial in which a difference in a coefficient of thermal expansionbetween the substrate 120 and the ferroelectric crystal 100 is 50% orless. More preferably, the substrate 120 and the ferroelectric crystal100 may be made of the same material (i.e. the same ferroelectricmaterial). By this, it is possible to prevent the damage, or theexfoliation or the like of the connecting surface between the substrate120 and the ferroelectric crystal 100 caused by the temperature changes,as described above. This is a great advantage as the material of aninformation recording medium on which information is recorded andreproduced by the application of a voltage, as described later. In otherwords, even if the temperature of the ferroelectric crystal 100increases and the ferroelectric crystal 100 expands due to the voltageapplication, since the substrate 120 also expands at the same rate orsubstantially the same rate, it is possible to prevent the damage, orthe exfoliation or the like of the connecting surface. Thus, it ispossible to appropriately use it as the information recording medium.

Moreover, not only the anodic bonding but also another direct connectingor bonding method (i.e. a method without using the adhesive 111, thebrazing material 113, or the like) may be used. Alternatively, it ispossible to use another method capable of appropriately connecting theelectrode 110 and the substrate 120 (the glass substrate 121).

In addition, the substrate 120 and the glass substrate 121 in FIG. 4 toFIG. 7 are preferably thicker than the ferroelectric crystal 100. Bythis, it is possible to minimize a loss of the ferroelectric crystal100. At the same time, the substrate 120 or the glass substrate 121 canbe used as the support of the thinned ferroelectric crystal 100.

Moreover, if a ferroelectric substance made of the same material as thatof the ferroelectric crystal 100 is used, a positional relationship ofcrystal orientation between the substrate 120 and the ferroelectriccrystal 100 may be the same.

Next, FIG. 8 shows the “polishing process” in FIG. 1. As shown in FIG.8, out of the ferroelectric crystal 100 after connecting the substrate120, the surface on the side that the electrode 110 etc. are not formed(one surface side) is grinded or polished by surface grinding or ageneral rotary board or the like. The ferroelectric crystal 100 isthinned to about 10 μm by the grinding or polishing. By this, it ispossible to reduce a portion of the ferroelectric crystal 100 to beetched in the subsequent etching process, to thereby improve theefficiency of the thinning operation.

Incidentally, the parallelism of the surface 101 of the ferroelectriccrystal 100 in the polishing process may be on the micrometer order. Inother words, in the present production method, the final parallelism ofthe surface 101 of the ferroelectric crystal 100 is on the several tensnanometer order or nanometer order; however, it is not necessarilyensure this highly accurate parallelism at the stage of the polishingprocess. That is because the parallelism on the several tens nanometerorder or nanometer order can be obtained in the subsequent etchingprocess.

Incidentally, in the polishing process, it is preferable to remove anaffected layer on the surface of the ferroelectric crystal 100 by theCMP (Chemical Mechanical Polishing) method after the mechanicalpolishing. By this, it is possible to reduce uneven etching in thesubsequent etching process and increase a processing accuracy andprocessing speed.

Next, FIG. 9 to FIG. 11 show the “etching process” in FIG. 1.

At first, FIG. 9 show a voltage-application etching apparatus used inthe etching process. As shown in FIG. 9(a), a voltage-applicationetching apparatus 200 in the embodiment includes: an acid-resistantcontainer 201; O rings 202; an electrode 203; an etching solution 204; apower supply 205; an electrode outgoing line 206; and fixtures 207.Then, the voltage-application etching apparatus 200 is used to performwet etching with respect to the ferroelectric crystal 100.

The acid-resistant container 201 is an etching tank filled with theetching solution 204 and functions as a fixed unit for fixing andholding the ferroelectric crystal 100 and the like. The acid-resistantcontainer 201 is directly exposed to the etching solution 204, so thatit preferably includes an insulating, acid-resistant material, such asTeflon (registered trademark).

Then, so as to connect the electrode outgoing line 206 and the powersupply 205, a path which the electrode outgoing line 206 can passthrough is formed in the container 201.

The O ring 202 is one specific example of the “sealing device” of thepresent invention, and prevents the leakage of the etching solution 204,not to expose the etching solution 204 to the surface other than thesurface 101 of the ferroelectric crystal 100. In addition, the O ring202 electrically insulates the electrode 110 and the electrode 203. TheO rings 202 are disposed along the periphery of the surface 101 to beclosely attached to the periphery. Moreover, the O ring 202 is alsodirectly exposed to the etching solution 204, so that it preferablyincludes an acid-resistant material.

The electrode 203 is one specific example of the “first connectingdevice” of the present invention, and is an electrode for applying avoltage to the surface 101 of the ferroelectric crystal 100 through theetching solution 204. The electrode 203 is directly exposed to theetching solution 204, so that it is preferably a metal electrode havingacid-resistance, such as Pt.

Moreover, as the other electrode used for the voltage application foretching, it is possible to use the electrode 110 (or the conductive baselayer 112) formed on the surface 102 of the ferroelectric crystal 100.Therefore, the voltage-application etching apparatus 200 is preferablyconstructed to connect the electrode 110 and the power supply 205. Forexample, as shown in FIG. 9(a), a leading line or the like (e.g. theelectrode outgoing line 206) may be provided, from the electrode 110,for one portion of the acid-resistant container 201, and the leadingline may be connected to the power supply 205.

The etching solution 204 includes a strong acid, such as hydrofluoricacid, and chemically erodes the surface 101 of the ferroelectric crystal100, to thereby make the ferroelectric crystal 100 with the desiredthickness. The etching tank constructed from the acid-resistantcontainer 201 is filled with the etching solution 204.

The power supply 205 applies a D.C. (Direct Current) voltage between theelectrode 110 and the electrode 203 to thereby control the progress ofthe etching. Particularly in the embodiment, by setting a voltagesupplied from the power supply 205 to a predetermined value in advance,it is possible to thin the ferroelectric crystal 100 to the desiredthickness. A relationship of the numerical values will be discussed indetail later (refer to FIG. 10).

Moreover, there may be provided a controlling device for setting orcontrolling various parameters (e.g. a voltage value, a current value,an application time length, polarity, a voltage waveform, etc.) of thepower supply 205. The controlling device may be able to automaticallycontrol the parameters by a CPU or the like, or may be able to manuallycontrol the parameters from the exterior, such as a keyboard and anoperation button.

The electrode outgoing line 206 is one specific example of the “secondconnecting device” of the present invention. It includes a contactmember which is in contact with the electrode 110, a copper wire, or thelike, for example, and applies a voltage supplied from the power supply205, to the electrode 110 which is fixed in the container 201 and whichis owned by the ferroelectric crystal 100 (including the substrate 120and the like). If the ferroelectric crystal 100 is fixed in thecontainer 201, the electrode outgoing line 206 preferably comes incontact with the electrode 205 at the same time of the fixing. By this,it is possible to apply a voltage to the electrode 205, from the powersupply 205, relatively easily.

The fixture 207 is used to fix the acid-resistant container 207.Moreover, it is preferably constructed to set up the ferroelectriccrystal 100 (including the substrate 120 and the like) to be etched, onthe voltage-application etching apparatus 200, and take out theferroelectric crystal 100 (including the substrate 120 and the like)after the etching, by releasing the fixing of the fixture 207 (or byunfixing).

Incidentally, as in a voltage-application etching apparatus 200 a shownin FIG. 9(b), it may be further provided with a heater 208, a stirringdevice 209, and a filtration pump 210.

The heater 208 is used to heat the etching solution 204. The heater 208preferably includes a conductive wire, such as a nichrome wire (whereinthe conductive wire is disconnected from the etching solution 204 and isinsulated (or isolated) from the etching solution having conductivity),and a power supply for supplying a current to the conductive wire, orthe like. By this, it is possible to increase the progression rate ofthe etching, to thereby perform the etching, more efficiently.

The stirring device 209 can stir the etching solution 204, regularly orirregularly, in order to keep the concentration of the etching solution204 uniform. For example, it preferably includes a motor and a propeller(or a stirring rod, etc.). By this, it is possible to uniformly promotethe etching with respect to the surface (101).

The filtration pump 210 filtrates impurities which are free in theetching solution 204. It preferably includes a filtration filter, a pumpwhich takes in the etching solution and opens after the filtration, orthe like. By this, along with the progress of the etching, it ispossible to remove the impurities which are free in the etching solution204 and perform the effective etching with respect to the surface (101).

In addition, if the etching solution 204 is heated by the heater 208,the evaporation of the etching solution 204 is promoted. Therefore,there may be provided a refluxing device for refluxing the evaporatedetching solution 204 to the etching tank again. The refluxing devicepreferably includes a vapor collecting device for collecting theevaporated etching solution 204 and a cooling/liquefying device forcooling and liquefying the collected vapor, or the like. Moreover, ifthe etching solution 204 is heated, there may be provided a temperaturemeasuring device, such as a mercury thermometer and a thermocouplethermometer, which is used to control the temperature of the etchingsolution 204.

Next, FIG. 10 show the procedure of the progress of the etching process.Incidentally, in the explanation of the procedure in FIG. 10, it isassumed that Z-cut LiTaO₃ is used as the ferroelectric crystal 100.Moreover, in FIG. 10, the voltage-application etching apparatus 200shown in FIG. 9 is simply illustrated. It is assumed that what isactually exposed to the etching solution 204 is only the surface 101 ofthe voltage-application etching apparatus 200 and that the electrode 110and the electrode 203 are insulated.

As shown in FIG. 10(a), the polarization direction of the ferroelectriccrystal 100, located in the etching solution 204, is set in advance tobe in a direction directed from the surface 101 to the surface 102. As aresult, the surface 101 exposed to the etching solution 204 is a minussurface. In LiTaO₃, the minus surface has a greater etching rate than aplus surface. Thus, the etching quickly proceeds on the surface 101.

Moreover, as shown in FIG. 10(a), in performing the etching process, theelectrode 110 is connected to the anode side of the power supply 205,and the electrode 203 is connected to the cathode side of the powersupply 205, to thereby drive the power supply 205. By this, apredetermined voltage is applied to the ferroelectric crystal 100located in the etching solution 204, to thereby form an electric filedin the ferroelectric crystal 100. At this time, the polarizationdirection of the ferroelectric crystal 100 and the electric field formedin the ferroelectric crystal 100 have opposite polarity.

If the predetermined voltage applied to the ferroelectric crystal 100 is“V_(DC)”, a coercive electric field of LiTaO₃ as being the ferroelectriccrystal 100 is “E_(C)”, and the thickness set on the ferroelectriccrystal 100 is the “desired thickness d”, V_(DC)=E_(C)×d. In otherwords, the voltage of the power supply 205 is set in order to apply thevoltage V_(DC) to the ferroelectric crystal 100. Specifically, thecoercive electric field E_(C) of LiTaO₃ is 22 kV/mm, so that if thedesired thickness d is 100 nm, for example, then, a voltage V is 2.2V.

At the start of the etching process, the etching has not proceeded yet.Thus, the thickness of the ferroelectric crystal 100 is the thicknessimmediately after the polishing process (e.g. 10 μm). Therefore, at thecurrent time point, even if the voltage V_(DC) of 2.2V is applied to theferroelectric crystal 100, the polarization direction is not reversed.

As the etching proceeds, the ferroelectric crystal 100 becomes thinner.Then, as shown in FIG. 10(b), when the thickness of the ferroelectriccrystal 100 reaches the desired thickness d, the polarization directionof the ferroelectric crystal 100 is reversed in a portion correspondingto the thickness d (i.e. a portion shown as 100 b in FIG. 10(b)). Thisis because if the thickness of the ferroelectric crystal 100 reaches thedesired thickness d, the electric field generated in the ferroelectriccrystal 100 by the voltage V_(DC), which is currently applied to theferroelectric crystal 100, exceeds the coercive electric field of theferroelectric crystal 100.

Here, as described above, in LiTaO₃, the plus surface has a much smalleretching rate than the minus surface. Thus, even if the etching solution204 is a solution which can provide an appropriate etching rate withrespect to the minus surface, it only provides an extremely smalletching rate, nearly equal to being not etched, with respect to the plussurface. In other words, in practice, it can be said that the plussurface is not etched by the etching solution 204. Therefore, if thereverse of the polarization direction of the ferroelectric crystal 100changes the portion of the surface 101 which has been the minus surface,to the plus surface 100 b, the progress of the etching is practicallystopped in the portion. As a result, the progress of the etching isstopped in the desired thickness d.

Therefore, if the coercive electric field E_(C) of the ferroelectriccrystal 100 is learned in advance, if the desired thickness d isdetermined in advance, and if the voltage V_(DC) of the power supply 205is set in advance on the basis of the above E_(C) and V_(DC), it ispossible to etch the ferroelectric crystal 100, highly accurately, untilreaching the desired thickness d.

Even some dispersion or variation of the progress of the etching maycause a time difference for the ferroelectric crystal 100 to reach thedesired thickness d. However, for example, the progress of the etchingis stopped in the portion 100 b which has already reached the desiredthickness d, the portion 100 b maintains the desired thickness d whilethe etching progresses in a portion 100 a in which the desired thicknessis not reached.

Before long, the thickness of the entire portion of the ferroelectriccrystal 100 becomes the desired thickness d, and the progress of theetching is completely stopped. As a result, the ferroelectric crystal100 has the thickness d over the entire surface, and the entire surfacebecomes flat or even.

As described above, according to the production method in theembodiment, even in the case of the ferroelectric crystal wafer having alarge area, it is possible to produce the ferroelectric thin film whichis extremely thin and which has a uniform thickness, by appropriatelyadjusting or controlling a voltage in the etching. Then, by using theferroelectric thin film produced in this manner as a material, it ispossible to mass-produce the ferroelectric recording medium.

Incidentally, since the etching solution 204 or the like has a slowprogression rate of the etching (i.e. a small etching rate), it ispreferable to heat the etching solution 204 to about 80 degrees Celsius,for example, by using the heater 208.

Incidentally, in FIG. 10, LiTaO₃ is used for the explanation; however,even in a ferroelectric crystal other than LiTaO₃ (e.g. theabove-mentioned LiNbO₃, etc.), it is possible to thin it to the desiredthickness if it is a ferroelectric crystal having a difference in theetching rate, due to the difference of plus (positive) and minus(negative) in the polarization.

Incidentally, the power supply 205 may supply not only a DC voltage butalso a pulse voltage, as shown in FIG. 11. In the case of the supply ofthe pulse voltage, it is possible to improve the control of the etching(e.g. fine processability, etching speed, etc.) by that theabove-mentioned controlling device adjusts or controls the strength ofthe pulse voltage, a pulse width, or a duty ratio, or the like, asoccasion demands.

In particular, if the DC voltage is applied, there is a possibility toincrease the etching rate of a surface which is originally difficult tobe etched (i.e. a surface having a plus (positive) polarizationdirection in the case of the ferroelectric crystal 100 as being LiTaO₃)because of an electrochemical action on the ferroelectric crystal 100.However, by applying the pulse voltage as shown in FIG. 11, it ispossible to apply a voltage appropriate enough not to produce theelectrochemical action, and it is possible to perform more appropriateetching.

Incidentally, even in the case where the pulse voltage is supplied, thecontrolling device may be provided, as in the case where above-mentionedDC voltage is supplied. For example, the controlling device may be ableto control various parameters of the power supply 205 (e.g. the strengthof the pulse voltage, a pulse width, or a duty ratio, or the like inaddition to the above-mentioned various parameters).

Moreover, various apparatuses used in the ferroelectric thin-filmproduction method in the above-mentioned embodiment can be combined tobe a thin-film producing apparatus. In other words, a sputteringapparatus, an anodic bonding apparatus, and a voltage-applicationetching apparatus are combined to be the thin-film producing apparatus,and by using the thin-film producing apparatus, it is possible toproduce a thin film including a ferroelectric crystal having theabove-mentioned various benefits.

Next, an example is given in the case where a ferroelectric crystalthin-film substrate produced in the ferroelectric thin-film productionmethod in the above-mentioned embodiment is applied to the ferroelectricrecording medium.

As shown in FIG. 12, the ferroelectric crystal thin-film substrateproduced in the ferroelectric thin-film production method in theembodiment can be used as a disc-shaped dielectric recording medium 300,for example. Namely, the ferroelectric crystal 100 is a recording layer,and the electrode 110 is used as the electrode of the dielectricrecording medium 300 as it is, and the substrate 120 can be used as thesupport of the dielectric recording medium 300 as it is. Incidentally,in FIG. 12, the adhesive 111 or the like is omitted. Moreover, theferroelectric crystal thin-film substrate having a large area andproduced in the ferroelectric thin-film production method in theembodiment can be cut into a rectangle and used as a recording mediumfor x-y scan recording/reproduction using a multi-probe.

As shown in FIG. 13, the information is recorded by the localpolarization direction of the ferroelectric crystal 100 of theferroelectric recording medium 300. For example, if the data bit of theinformation is “1”, it is recorded as an upward polarization direction.If the data bit of the information is “0”, it is recorded as a downwardpolarization direction. The information is recorded by applying anelectric filed which exceeds the actual coercive electric field onto theferroelectric crystal 100. Specifically, a voltage pulse signalcorresponding to the information (data bit row) is applied between aprobe 310 and the electrode 110 so as to form an electric filed whichexceeds the actual coercive electric field on the ferroelectric crystal100 only if the pulse is at high level. Then, the application of thevoltage pulse signal is performed while a positional relationshipbetween the probe 310 and the electrode 110 is changed in anarrow-pointing direction in FIG. 13. By this, the polarization directionis reversed only in the portion where the electric field which exceedsthe coercive electric field is formed, which causes the information tobe recorded into the ferroelectric crystal 100.

In the ferroelectric crystal 100 produced in the above-mentionedproduction method, the polarization directions are all oriented in thesame direction, immediately after the production. Then, the polarizationdirections are perpendicular to the recording surface. Therefore, theferroelectric crystal 100 can be immediately used as the ferroelectricrecording medium 300 into which the information is recorded by thereverse of the polarization direction. As a result, there is anadvantage that it is unnecessary to perform a format operation of alignor uniform the polarization directions in using the dielectric recordingmedium 300.

Then, the ferroelectric crystal 100 produced in the above-mentionedproduction method is thin, about 100 nm in thickness. Therefore, it ispossible to reduce a voltage to reverse the polarization direction ofthe ferroelectric crystal 100; namely, the voltage of the recordingvoltage pulse to record the information, and thus it is possible toreduce the power consumption of the recording apparatus.

Moreover, the ferroelectric crystal 100 produced in the above-mentionedproduction method is uniform in thickness. Therefore, it is possible toprovide a constant voltage to reverse the polarization direction of theferroelectric crystal 100; namely, the constant voltage of the recordingvoltage pulse to record the information.

Incidentally, the drawings used for the explanation of the embodimentsof the present invention embodies the constitutional elements or thelike of the ferroelectric thin-film production method, thevoltage-application etching apparatus, and the thin film of the presentinvention, only for the purpose of explaining technical ideas thereof.The shape, size, position, connection relationship, and the like of thevarious constitutional elements or the like are not limited to thedrawings.

In the present invention, various changes may be made, if desired,without departing from the essence or spirit of the invention which canbe read from the claims and the entire specification. The constitutionalelements or the like of the ferroelectric thin-film production method,the voltage-application etching apparatus, and the thin film, all ofwhich involves such changes, are also intended to be within thetechnical scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the ferroelectric thin-filmproduction method, the voltage-application etching apparatus used in theferroelectric thin-film production method, and the ferroelectric crystalthin-film substrate including the thin film produced in theferroelectric thin-film production method.

1. A ferroelectric thin-film production method comprising: an etchingprocess of dipping one surface in an etching solution to thereby etchthe one surface, with respect to a ferroelectric crystal which has theone surface and another surface that face each other and in which anetching rate of the one surface is greater than an etching rate of theanother surface in such a condition that polarization directions areoriented in one direction; and a voltage applying process of applying apredetermined voltage between the one surface and the another surface.2. The ferroelectric thin-film production method according to claim 1,wherein the ferroelectric crystal is a single crystal wafer of aferroelectric substance.
 3. The ferroelectric thin-film productionmethod according to claim 1, wherein the ferroelectric crystal includesat least one of LiNb_(x)Ta_(1−x)O₃ (0≦x≦1), M: LiNb_(x)Ta_(1−x)O₃(0≦x≦1, M is a doping material), andK₃Li_(2−x)(Nb_(1−y)Ta_(y))_(5+x)O_(15+2x).
 4. The ferroelectricthin-film production method according to claim 1, wherein if a desiredthickness of the ferroelectric crystal is d and a coercive electricfield of the ferroelectric crystal is Ec, the predetermined voltage isEc×d.
 5. The ferroelectric thin-film production method according toclaim 1, wherein the voltage is a direct current voltage.
 6. Theferroelectric thin-film production method according to claim 1, whereinthe voltage is a pulse voltage.
 7. The ferroelectric thin-filmproduction method according to claim 4, wherein the desired thickness isless than 1 μm.
 8. The ferroelectric thin-film production methodaccording to claim 1, wherein the etching solution includes hydrofluoricacid.
 9. The ferroelectric thin-film production method according toclaim 1, wherein the etching solution has conductivity.
 10. Theferroelectric thin-film production method according to claim 1, whereinsaid etching process includes a heating process of heating the etchingsolution.
 11. The ferroelectric thin-film production method according toclaim 1, wherein said etching process includes a stirring process ofstirring the etching solution.
 12. The ferroelectric thin-filmproduction method according to claim 1, wherein said etching processincludes a filtrating process of filtrating the etching solution. 13.The ferroelectric thin-film production method according to claim 1,wherein the ferroelectric thin-film production method comprises: anelectrode forming process of forming an electrode in a film shape on theanother surface; a substrate connecting process of connecting theelectrode and a substrate; and a polishing process of polishing the onesurface of the ferroelectric crystal, and said etching process and saidvoltage applying process are performed after said electrode formingprocess, said substrate connecting process and said polishing process.14. The ferroelectric thin-film production method according to claim 13,wherein the substrate has a difference in a coefficient of thermalexpansion of 50% or less, as compared to the ferroelectric crystal. 15.The ferroelectric thin-film production method according to claim 13,wherein the substrate includes a same material as that of theferroelectric crystal.
 16. The ferroelectric thin-film production methodaccording to claim 13, wherein the electrode and the substrate areconnected by providing an adhesive layer therebetween, in said substrateconnecting process.
 17. The ferroelectric thin-film production methodaccording to claim 16, wherein a conductive base layer which is highlyadhesive is provided at least one of between the adhesive layer and theelectrode, and between the adhesive layer and the substrate, in saidsubstrate connecting process.
 18. The ferroelectric thin-film productionmethod according to claim 13, wherein the substrate is a glasssubstrate, and the electrode and the substrate are directly connected byanodic bonding, in said substrate connecting process.
 19. Theferroelectric thin-film production method according to claim 13, whereina glass film is provided between the electrode and the substrate, andthe electrode and the substrate are connected by anodic bonding, in saidsubstrate connecting process.
 20. The ferroelectric thin-film productionmethod according to claim 13, wherein a smoothing process is performedto the one surface after the one surface is polished.
 21. Avoltage-application etching apparatus used in an etching process and avoltage applying process in a ferroelectric thin-film production methodcomprising: an etching process of dipping one surface in an etchingsolution to thereby etch the one surface, with respect to aferroelectric crystal which has the one surface and another surface thatface each other and in which an etching rate of the one surface isgreater than an etching rate of the another surface in such a conditionthat polarization directions are oriented in one direction; and avoltage applying process of applying a predetermined voltage between theone surface and the another surface, said voltage-application etchingapparatus comprising: a container to dip the one surface in the etchingsolution; a sealing device for sealing a portion other than the onesurface from the etching solution; a power supply for applying thepredetermined voltage between the one surface and the another surface; afirst connecting device for electrically connecting one output terminalof said power supply with the one surface; and a second connectingdevice for electrically connecting another output terminal of said powersupply with the another surface.
 22. The voltage-application etchingapparatus according to claim 21, wherein said sealing device is anacid-resistant O ring which is pressed onto a periphery portion on theone surface or an outer edge of the one surface.
 23. Thevoltage-application etching apparatus according to claim 21, furthercomprising a heating device for heating the etching solution.
 24. Thevoltage-application etching apparatus according to claim 21, furthercomprising a stirring device for stirring the etching solution.
 25. Thevoltage-application etching apparatus according to claim 21, furthercomprising a filtrating device for filtrating the etching solution. 26.A ferroelectric crystal thin-film substrate comprising: a substrate; anelectrode formed on the substrate; and a ferroelectric crystal which isformed on the electrode and which is less than 1 μm in thickness,wherein an area size of an entire surface of the substrate is equal toor greater than 10 mm².
 27. The ferroelectric crystal thin-filmsubstrate according to claim 26, wherein the substrate is a glasssubstrate, and the substrate and the electrode are directly connected byanodic bonding.
 28. The ferroelectric crystal thin-film substrateaccording to claim 26, wherein in the ferroelectric crystal,polarization directions are perpendicular to the surface and oriented ina same direction.
 29. A ferroelectric crystal wafer comprising: asubstrate; an electrode formed on the substrate; and a ferroelectriccrystal which is formed on the electrode and which is less than 1 μm inthickness.
 30. The ferroelectric crystal wafer according to claim 29,wherein the substrate is a glass substrate, and the substrate and theelectrode are directly connected by anodic bonding.
 31. Theferroelectric crystal wafer according to claim 29, wherein in theferroelectric crystal, polarization directions are perpendicular to thesurface and oriented in a same direction.